Aircraft typically include a plurality of flight control surfaces that, when controllably positioned, guide the movement of the aircraft from one destination to another. The number and type of flight control surfaces included in an aircraft may vary, but typically include both primary flight control surfaces and secondary flight control surfaces. The primary flight control surfaces are those that are used to control aircraft movement in the pitch, yaw, and roll axes, and the secondary flight control surfaces are those that are used to influence the lift or drag (or both) of the aircraft. Although some aircraft may include additional control surfaces, the primary flight control surfaces typically include a pair of elevators, a rudder, and a pair of ailerons, and the secondary flight control surfaces typically include a plurality of flaps, slats, and spoilers.
The positions of the aircraft flight control surfaces are typically controlled using a flight control surface actuation system. The flight control surface actuation system, in response to position commands that originate from either the flight crew or an aircraft autopilot, moves the aircraft flight control surfaces to the commanded positions. In most instances, this movement is effected via actuators that are coupled to the flight control surfaces. Though unlikely, it is postulated that a flight control surface actuator could become jammed, uncontrollably free, or otherwise inoperable. Thus, some flight control surface actuation systems are implemented with redundant (e.g., two or more) actuators coupled to a single flight control surface.
Flight control surface actuation systems that have two or more actuators coupled to a single flight control surface typically implement one of two operational configurations—an active-standby configuration or an active-active configuration. With the active-standby (or active-standby-standy) configuration, one actuator is actively powered while the other one (or two) are in a standby mode. With the active-active (or active-active-active) operational configuration, all of the actuators are simultaneously powered. This latter operational configuration provides certain advantages over the active-standby (or active-standby-standy) configuration. Specifically, it allows each individual actuator to be sized relatively smaller as compared to the actuators used to implement the active-standby (active-standby-standby) configuration. Additionally, there is no need for any redundancy management. It is noted, however, that the active-active (or active-active-active) operational mode does present the potential for a resultant force fight between the active actuators.
The force-fight results from the fact that the actuators, position sensors, control electronics, and mechanical components have independent, unique tolerances. Although installation and surface position rigging can reduce some of the differences between two channels, these differences can result in one channel attempting to position the flight control surface to a different position than the other channel(s). The resultant effect is torsion moment on the flight control surface as the two neighbouring channels compete with each other to move the flight control surface to different positions. This torsion moment introduces stress to the flight control surface and a resulting fatigue accumulation. Designing flight control surfaces to withstand the worst-case stress and fatigue that could occur in the active-active (or active-active-active) operational configuration would result in additional weight, and associated its costs.
Hence, there is a need for a system and method of preventing, or at least mitigating, the resultant force fights that can occur between actuators when flight control surface actuation system channels are configured in an active-active (or active-active-active) operational configuration without relying on undesirably heavy flight control surfaces. The present invention addresses at least this need.